Polyolefin porous films have been used in battery separators, electrolytic capacitors and the like. In particular, they have been used as separators insoluble in organic solvents and stable to electrolytes or electrode active materials in lithium batteries. Recently, it has been demanded for lithium battery separators to improve battery characteristics, safety and productivity. In particular, it is very important to balance the battery characteristics and the safety at high levels. In order to improve the battery characteristics, it has been required to optimize the pore size, porosity, ion permeability and the like of the battery separators used in various battery systems. Further, in terms of the safety, importance is attached to improvement of film strength and a decrease in thermal shrinkage ratio, in order to prevent the occurrence of accidents such as ignition at the time when electrodes are short-circuited to rise the temperature of the insides of the batteries. From these, various proposals have been made in order to decrease the thermal shrinkage ratio while maintaining the balance of the pore size, the porosity, the ion permeability and the film strength.
For example, there is proposed a method for producing a polyolefin porous film including melt-kneading a solution composed of an ultrahigh molecular weight polyolefin having a weight average molecular weight of 500,000 or more, high-density polyethylene having a weight average molecular weight of 10,000 to less than 500,000 and a solvent, followed by extruding and cooling, drawing the resulting gel-like molded product, removing the solvent from the resulting drawn product, and performing a heat set process after drying (JP-A-2001-172420 and JP-A-2001-192487), and an example of use as the lithium battery separator is also mentioned.
Patent Document 1: JP-A-2001-172420
Patent Document 2: JP-A-2001-192487